Electrophotographic printers and cartridges typically use either a mechanically milled toner or a chemically prepared toner (‘CPT’). Chemically prepared toner can be a toner derived from using a suspension polymerization method, an emulsion agglomeration (‘EA’) method, or an aggregation method. Independent of the method of preparation, toner flow properties and print quality metrics can be suitably manipulated by use of extra particulate additives (‘EPA’s) to the toner particle surface. EPAs help improve the toner flow behavior, lower or eliminate the tendency to brick or cake under high temperature and/or humidity, improve transfer of toner from a photoreceptor to paper or an image transfer member, transfer between an image transfer member and paper, or regulate the toner charge across various environments (ie, varying temperature and humidity) and improve print quality.
Whereas most of the toner formulation is printed on a document, a small amount of toner is lost as waste. Hence there is a desire to minimize waste toner and therefore maximize the toner usage efficiency. Toner usage efficiency is described as the ratio of toner on a printed page to total toner used. Similarly, waste toner will be herein after referred to as “toner in the cleaner” or “toner-to-cleaner (TTC)”.
Several EPAs have been employed in the surface treatment of toner. These EPAs include various inorganic oxides such as silicon dioxide also known as silica, titanium dioxide also known as titania, aluminum oxide, and composite mixtures of titania, silica and/or alumina. Further metal soaps have also been used to improve the transfer efficiency of a toner.
Inorganic oxides may be obtained using a fuming process or a colloidal process. Fumed silica, also known as pyrogenic silica, is produced in a flame. This type of silica consists of microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles. In a typical case, fumed silica is produced by pyrolysis of silicon tetrachloride.
Inorganic oxides such as silica, titania, alumina etc., can vary in their primary particle size from about a 5 nm to several micrometers. Moreover to achieve uniform print quality across different type of environments, inorganic oxides are surface treated with various treatments such as organosilanes and silicone oil. The extent of surface treatment of the hydroxyl groups in an inorganic oxide can also be varied. In regards to the primary particle size of then silica, the toner flow can be significantly improved by use of a smaller primary particle size silica, usually about 5 nm-15 nm in combination with a large primary particle size such as 40 nm-250 nm. This larger sized silica serves as a useful ‘spacer’. Spacers are effective in keeping individual toners apart and hence can improve the storage stability. Silicas with a primary particle size of about 100 nm has been used in CPT toners to be effective spacers. The large silica described as a spacer is typically prepared by a sol-gel or colloidal process. Whereas the medium size silica, about 30 nm-60 nm primary particle size help with toner flow, they are ineffective spacers, and the large silica while functioning as a spacer requires to be used at higher concentrations or levels to help with toner flow. Hence there is a need for a silica that can help both with toner flow and also act as a suitable spacer between surface treated toner particles.
The inventor has discovered that the use of 60 nm-100 nm fumed silica, more preferably a 70 nm-80 nm silica, prepared from a fuming process the helps impart the needed optimum spacer behavior and significantly improves the toner usage efficiency, thereby resulting in a lower toner-to-cleaner or waste toner. Having this optimum spacing behavior generates a toner formulation having less toner waste, increases toner usage efficiency and significantly reduces toner consumption without impacting image quality and charge characteristics.
The present disclosure is directed at a toner formulation which generates less toner waste, increases toner usage efficiency and significantly reduces toner consumption without impacting image quality and charge characteristics by providing extra particular agents including a specific mixture of silicas—namely a first fumed silica having a primary particle size of about 30-50 nm and a second fumed silica having a primary particle size of 70 nm-80 nm to the toner, and in particular, to the toner particle surface. The toner particles may be prepared by a chemical process, such as suspension polymerization or emulsion aggregation. In one example, the toner particles may be prepared via an emulsion aggregation procedure, which generally provides resin, colorant and other additives. More specifically, the toner particles may be prepared via the steps of initially preparing a polymer latex from vinyl type monomers, such as acrylate based monomers or styrene-acrylate base copolymers, in the presence of an ionic type surfactant. The polymer latex so formed may be prepared at a desired molecular weight distribution (MWD=Mw/Mn) and may, for example, contain both relatively low and relatively high molecular weight fractions to thereby provide a relatively bimodal distribution of molecular weights. Pigments may then be milled in water along with a surfactant that has the same ionic charge as that employed for the polymer latex. Release agent (e.g., a wax or mixture of waxes) including olefin type waxes such as polyethylene may also be prepared in the presence of a surfactant that assumes the same ionic charge as the surfactant employed in the polymer latex. Optionally, one may include a charge control agent.
The polymer latex, pigment dispersion and wax dispersion may then be mixed and the pH adjusted to cause flocculation. For example, in the case of anionic surfactants, acid may be added to adjust pH to neutrality. Flocculation therefore may result in the formation of a gel where an aggregated mixture may be formed with particles of about 1-2 μm in size.
Such mixture may then be heated to cause a drop in viscosity and the gel may collapse and relative loose (larger) aggregates, from about 1-25 μm, may be formed, including all values and ranges therein. For example, the aggregates may have a particle size between 3 μm to about 15 μm, or between about 4 μm to about 10 μm. In addition, the process may be configured such that at least about 80-99% of the particles fall within such size ranges, including all values and increments therein. Base may then be added to increase the pH and reionize the surfactant or one may add additional anionic surfactants. The temperature may then be raised to bring about coalescence of the particles. Coalescence is referenced to fusion of all components. The toner may then be removed from the solution, washed and dried.
It is also contemplated herein that the toner particles may be prepared by a number of other methods including mechanical methods, where a binder resin is provided, melted and combined with a wax, colorant and other optional additives. The product may then be solidified, ground and screened to provide toner particles of a given size or size range.
The resulting toner may have an average particle size in the range of 1 μm to 25 μm. The toner may then be treated with a blend of extra particulate agents, including hydrophobic fumed alumina, hydrophobic fumed small silica sized less than 20 nm, medium silica sized 40 nm to 50 nm, large fumed silica sized 70 nm to 80 nm, and titania. Treatment using the extra particulate agents may occur in one or more steps, wherein the given agents may be added in one or more steps.
The alumina (Al2O3) that may be used herein may have an average primary particle size in the range of 5 nm to 100 nm, including between 7 nm to 50 nm (largest cross-sectional linear dimension) or between 7 nm to 25 nm. In addition, the alumina may be surface treated with an inorganic/organic compound which may then improve mixing (e.g., compatibility) with organic based toner compositions. For example, the alumina may include a silane coating or other coatings, such as chloro(dimethyl)octylsilane, dimethoxy(methyl)octylsilane, or methoxy(dimethyl)octylsilane. The alumina may be present in the range of 0.01% to 1.0% by weight of the toner composition, including in the range of 0.10% to 0.50% by weight. An example of the aluminum oxide may be that available from Evonik Corporation under the tradename AEROXIDE and product number C 805.
Referring again to the extra-particulate agents that may be used herein, small silica may be understood as silica (SiO2) having an average primary particle size in the range of 2 nm to 20 nm, or between 5 nm to 15 nm (largest cross-sectional linear dimension) prior to any after treatment, including all values and increments therein. The small silica may be present in the toner formulation as an extra particulate agent in the range of 0.01% to 3.0% by weight of the toner composition, such as 0.1% to 1.0% by weight, including all values and increments therein. In addition, this small silica may be treated with hexamethyldisilazane. An exemplary silica may be available from Evonik Corporation under the tradename AEROSIL and product numbers R812.
Medium sized silica may be understood as silica having a primary particle size in the range of 30 nm to 60 nm, or between 40 nm to 50 nm, prior to any after treatment, including all values and increments therein. Primary particle size may be understood as the largest linear dimension through a particle volume. The medium sized silica may be present in the toner formulation as an extra particulate agent in the range of 0.1% to 2.0% by weight of the toner composition, including all values and increments in the range of 0.1% to 2.0% by weight. The medium sized silicas may also be treated with surface additives that may impart different hydrophobic characteristics or different charges to the silica. For example, the silica may be treated with hexamethyldisilazane (silane), polydimethylsiloxane (silicone oil), etc. Exemplary silicas may be available from Evonik Corporation under the tradename AEROSIL and product numbers RX-50 or RY-50.
Large fumed silica may be understood as silica having a primary particle size in the range of 70 nm to 120 nm, or between 70 nm to 80 nm, prior to any after treatment, including all values and increments therein. The large fumed silica may be present in the toner formulation as an extra particulate agent in the range of 0.1% to 2 weight %, for example in the range of 0.25 weight % to 1 weight % of the toner composition. The large fumed silica may also be treated with surface additives that may impart different hydrophobic characteristics or different charges to the silica. For example, the large colloidal silica may be treated with hexamethyldisilazane, polydimethylsiloxane, dimethyldichlorosilane, and combinations thereof, wherein the treatment may be present in the range of 1 weight % to 10 weight % of the silica. Exemplary large silicas may be available from Evonik Corporation under the trade name VPRY40S or VPRX40S.
In addition, titania (titanium-oxygen compounds such as titanium dioxide) may be added to the toner composition as a extra particulate additive. The titania may be present in the formulation in the range of about 0.01% to 3.0% by weight of the toner formulation, such as 0.1% to 1.0%. The titania may include a surface treatment, such as aluminum oxide. The titania particles may have a mean particle length in the range of 0.1 μm to 3.0 μm, such as 0.5 μm-2.0 μm and a mean particle diameter in the range of 0.01 μm to 0.2 μm, such as 0.13 μm. An example of titania contemplated herein may include FTL-110 available from ISK USA. Other contemplated titanias may include those available from DuPont; Kemira of Finland under the product designation Kemira RODI or RDI-S; or Huntsman Pigments of Texas under the product name TIOXIDE R-XL.
The disclosed method to make the toner of the present invention operates to provide a finishing to toner particles, as more specifically described below. Such finishing may rely upon what may be described as a device for mixing, cooling and/or heating the particles which is available from Hosokawa Micron BV and is sold under the trade name “CYCLOMIX.” Such device may be understood as a conical device having a cover part and a vertical axis which device narrows in a downward direction. The device may include a rotor attached to a mixing paddle that may also be conical in shape and may include a series of spaced, increasingly wider blades extending to the inside surface of the cone that may serve to agitate the contents as they are rotated. Shear may be generated at the region between the edge of the blades and the device wall. Centrifugal forces may therefore urge product towards the device wall and the shape of the device may then urge an upward movement of product. The cover part may then urge the products toward the center and then downward, thereby providing a feature of recirculation.
The device as a mechanically sealed device may operate without an active air stream, and may therefore define a closed system. Such closed system may therefore provide relatively vigorous mixing and the device may also be configured with a heating/cooling jacket, which allows for the contents to be heated in a controlled manner, and in particular, temperature control at that location between the edge of the blades and the device wall. The device may also include an internal temperature probe so that the actual temperature of the contents can be monitored.
For example, conventional toner or chemically prepared toner (CPT) may be combined with one or more extra particulate additives and placed in the above referenced conical mixing vessel. The temperature of the vessel may then be controlled such that the toner polymer resins are not exposed to a corresponding glass transition temperature or Tg which could lead to some undesirable adhesion between the polymer resins prior to mixing and/or coating with the EPA material. Accordingly, the heating/cooling jacket may be set to a temperature of less than or equal to the Tg of the polymer resins in the toner, and preferably to a cooling temperature of less than or equal to about 25° C.
The conical mixing device with such temperature control may then be operated wherein the rotor of the mixing device may preferably be configured to mix in a multiple stage sequence, wherein each stage may be defined by a selected rotor rpm value (RPM) and time (T). Such multiple stage sequence may be particularly useful in the event that one may desire to provide some initial break-up of toner agglomerates. In addition, such initial first stage of mixing may be controlled in time, such that the conical mixer operates at such rpm values for a period of less than or equal to about 60 seconds, including all values and increments therein. Then, in a second stage of mixing, the rpm value may be set higher than the rpm value of the first stage, e.g., at an rpm value greater than about 500 rpm. Furthermore, the time for mixing in the second stage may be greater than about 60 seconds, and more preferably, about 60-180 seconds, including all values and increments therein. For example, the second stage may therefore include mixing at a value of about 1300-1350 rpm for a period of about 90 seconds. Following the above mentioned blending the toner with surface additives can be subjected to a screening step or a classifying step to remove any undesired large agglomerates or particles. It may be appreciated that following the screening or classifying step the toner can be placed in the conical mixer and further blended to achieve better adhesion of the surface additives to the toner surface.
It can therefore be appreciated that with respect to the mixing that may take place in the present invention, as applied to mixing EPA with toner, such mixing may efficiently take place in multiple stages in a conical mixing device, wherein EPA may be added in a first stage wherein the breaking of aggregates may be accomplished, followed by screening, and then additional EPA added before the toner is cooled. In addition, the temperature of the mixing process may again be controlled within such multiple staged mixing protocol such that the heating/cooling jacket and/or the polymer within the toner (as measured by an internal temperature probe) is maintained below its glass transition temperature (Tg).
It has been found that the mixing of toner particulate with extra particulate additive in the conical mixing device according to the above provides a relatively more uniform surface distribution of EPA.
The extra particulate additives (EPA) may serve a variety of functions, such as to modify or moderate toner charge, increase toner abrasive properties, influence the ability/tendency of the toner to deposit on surfaces, improve toner cohesion, or eliminate moisture-induced tribo-excursions. The extra particulate additives may therefore be understood to be a solid particle of any particular shape. Such particles may be of micron or submicron size and may have a relatively high surface area. The extra particulate additives may be organic or inorganic in nature. For example, the additives may include a mixture of two inorganic materials of different particle size, such as a mixture of differently sized fumed silica. The relatively small sized particles may provide a cohesive ability, e.g. ability to improve powder flow of the toner. The relatively larger sized particles may provide the ability to reduce relatively high shear contact events during the image forming process, such as undesirable toner deposition (filming).
The examples herein are for the purposes of illustration and are not intended to be exhaustive or to limit the invention to the formulations discussed herein.